Adding descriptions.

Server---Build-level-API.md

@@ -9,6 +9,11 @@ A new key available which can be used to interact with the server:
 val uiContext = taskKey[sbt.UIContext]("All user interaction should be driven through this interface.")
 ```
 
+Custom tasks and plugins will be able to interact with sbt clients via the `uiContext in Global` setting.  All interaction with users should occur over the UIContext.  Tasks should no longer attempt to read from System.in, as this will begin to throw exceptions.
+
+_TODO - Should we attempt for System.in compatibility for a bit?_
+
+
 # 2. A UI interface
 
 ```scala
@@ -28,8 +33,22 @@ sealed trait UIContext {
 private[sbt] abstract class AbstractUIContext extends UIContext
 ```
 
+This interface represents *all* the interaction that can be done with users.  Interaction with sbt clients can consist of the following:
+
+* Reading a single line of input (potentially where you hide the user keystrokes).  
+  _Example(s):  Reading the PGP passphrase when needed, or selecting the main class to run._
+* Prompting the user to confirm before continuing.
+  _Example(s):  Ensuring the user wants to create a tag before continuing the release script.
+* Sending structured events to sbt clients.
+  _Example(s):  Notifying your own Intellij Plugin the progress of your integration tests._
+
 # 3. Plugin API
 
+Plugins will, additionally, have the ability to tell the server how their build values can be serialized
+to interested clients.  While many clients are expected to just listen to `toString` values, it is expected
+that some plugins may wish for their own rich tooling support.  In this event, e.g. an IDE plugin could
+receive the complete structured data that results when a task is run.
+
 ```scala
 trait Plugin {
   ...
@@ -42,4 +61,3 @@ trait Plugin {
 }
 ```
 
-# TODO - More information about the changes...